Calling upon and assisting police officers are acts of public cooperation that link informal and formal mechanisms of social control. An in-depth study of seven London neighborhoods investigates the relationships between (a) cooperation with the police, (b) public trust in police fairness and effectiveness, and (c) public perceptions of everyday social regulation processes. Cooperation with the police is associated first with high levels of public trust in procedural fairness, second with confidence that local residents will intervene on behalf of the collective good, and third with heightened concerns about disorder and the loss of authority and discipline in society. We conclude with the idea that cooperation is shaped by trust in the police and is reinforced and challenged by a complex set of relational concerns. Moreover, by recognizing and supporting the function of the police to fight crime and administer justice, acts of cooperation both constitute and confer police legitimacy.